The present invention relates generally to a planar coil and a plane transformer, and more particularly to a planar coil and a planar transformer capable of operating with small power of 10 W or less. The present invention is also concerned with a process of fabricating a high-aspect conductive device comprising a plurality of fine yet thick conductive patterns arranged side by side at a narrow spacing, for instance, a planar coil, and a printed circuit board.
Planar coils are widely used in the form of biaxial actuators for digital audio disks, and power sources or signal sources for artificial satellites, and fine-pitch printed circuit boards are employed as general-purpose parts of portable terminal equipment, and high-density packaged electronic equipment. Planar coils are also incorporated in thin-film heads.
As planar coils, and printed circuit boards capable of functioning as precise devices are now in growing demand, an arrangement comprising a plurality of fine yet thick conductive patterns, i.e., so-called high-aspect conductive patterns arranged side by side at a narrow spacing is greatly needed. So far, such a high-aspect conductive device has been fabricated by a process comprising steps of depositing a conductive thin film on an insulating substrate, applying a negative photoresist on the conductive thin film to form a resist pattern in a conventional manner, etching an exposed region of the conductive thin film, and finally stripping away the resist pattern.
However, a serious problem with such a process is that while the conductive thin film is etched, an etching solution burrows its way into a portion of the conductive thin film covered with the resist pattern, causing removal of the conductor at that portion by dissolution. This in turn causes the section of the remaining conductor to assume on a trapezoidal shape, resulting in considerable increases in the spacing between conductive patterns.
To solve this problem, it has been proposed to use a process wherein a conductive thin film is etched to form a spiral pattern thereon, and the conductive thin film is thickly plated in a selective fashion making use of an electric resistance difference between an insulating substrate and the conductive thin film (JP-A 58-12315). However, such as when the spiral pattern is especially formed on the conductive thin film that is a plating primer film, there is an increase in the wiring resistance of the primer film because the thickness of the conductive thin film is very small. This then renders it impossible to increase plating currents, resulting in an unavoidable increase in the length of time needed for plating. In addition, it is impossible to increase the thickness of the conductive pattern because the growth rate of plating is usually lacking in anisotropy.
There has also been known a process wherein a thick resist pattern is formed on a metal thin film provided all over the surface of an insulating substrate, a high-aspect conductor is then formed by pattern plating, followed by the stripping-away of the resist, and the metal thin film between lines is finally stripped away by dry etching such as ion milling. Since the upper limit to resist thickness is at most 50 xcexcm, the thickness of the resulting conductive pattern is barely about 40 xcexcm at most. To add to this, high-speed plating causes deformation of the resist wall due to its softness; there is no choice but to decrease the working current, with a working efficiency drop. Moreover, the need of special equipment incurs some extra expenses. Thus, many problems arise in putting this process to practical use.
When this process is applied to the fabrication of a planar coil, it is difficult to increase the thickness of a coil conductor itself. In addition, the formation of a conductive pattern by etching limits the spacing between filaments forming the coil conductor to at most about twice as large as the layer thickness of the coil conductor. In other words, the space factor of a coil conductive portion is too low to achieve favorable electric properties, because direct current resistance increases unavoidably.
One object of the invention is to provide a compact yet high-performance planar coil of low direct current resistance by increasing the layer thickness of a coil conductor and reducing the spacing between filaments forming the coil conductor, and a high-performance transformer using the same.
Another object of the invention is to provide a process of fabricating, with ease, a device comprising a plurality of fine yet thick conductive patterns, i.e., high-aspect conductive patterns arranged side by side at a narrow spacing, thereby providing a solution to various problems with conventional processes of fabricating high-aspect conductive devices.
As a result of study after study made to achieve a high-performance planar coil, the inventors have now found that each filament of a coil conductor is formed by anisotropic growth into a mushroomy shape in section, so that the coil conductor can have a height of at least 20 xcexcm with a spacing of up to 20 xcexcm between adjacent filaments, and hence the coil conductor can have improved electric properties due to an apparent space factor increase. This finding underlies the first aspect of the invention.
According to the first aspect of the invention, there is provided a planar coil comprising an insulating substrate and a 20 to 400-xcexcm thick coil conductive filament formed on one or both surfaces of said substrate at an aspect ratio (H/G) of at least 1 at an gap position thereof, optionally with a protective metal plating thin-film layer provided over a surface thereof, characterized in that said coil conductive filament has a mushroomy shape in section, with a width (L) of a head of said section being at least twice as large as a width (l) of a neck thereof and at most 1.5 times as large as a height of said head, and being at least twice as large as a minimum spacing (G) in section between adjacent coil conductive filaments. According to the first aspect of the invention, there is also provided a planar transformer obtained by laminating such planar coils one upon another with an insulating film interleaved therebetween, and sandwiching the whole of the thus obtained multilayer structure between low-profile ferromagnetic cores.
As a result of study after study made of how to fabricate a planar coil, the inventors have now found that a plurality of high-aspect conductive patterns, each having a narrow width yet a large thickness, can be arranged side by side at a narrow spacing by using a positive photoresist as a photoresist for the formation of a mask pattern on a conductive thin-film layer, thereby additionally stripping away the photoresist between the conductive patterns by active beam irradiation after plating, and selectively stripping away a plating primer thin-film layer between the coil conductors while a protective thin-film layer formed on the coil conductors remains intact. This finding underlies the second aspect of the invention.
According to the second aspect of the invention, there is provided a process of fabricating a high-aspect conductive device, characterized comprising, as shown in FIG. 5, a step (A) of providing an electrically conductive primer thin-film layer 21 on an insulating substrate 1, a step (B) of laminating a positive photoresist layer 61 on said plating primer thin-film layer 21, a step (C) of applying photolithography to said positive photoresist layer 61 to form a photoresist mask pattern of said positive photoresist layer 61, a step (D) of carrying out plating so that a coil conductive plating layer 31 of a mushroomy shape in section can swell up on an exposed region 21xe2x80x2 of said plating primer thin-film layer 21 and over regions 15 and 16 covered by said photoresist mask pattern in the vicinity of said exposed region, a step (E) of carrying out plating so that a protective metal thin-film layer 8 can be provided all over the coil conductive plating layer, a step (p) of irradiating an entire surface of a multilayer structure with an active beam and then developing said multilayer structure to strip away a photoresist mask pattern 62xe2x80x3 exposed under openings in said coil conductive plating layer, and a step (G) of stripping away only an exposed plating primer thin-film layer 21xe2x80x3 by selective etching, said steps (A) to (G) being successively carried out.
In the planar coil according to the first aspect of the invention, the thickness of the conductor can be increased while the spacing between coil conductive filaments is kept small, so that the direct current resistance of the planar coil can be reduced. This planar coil may be used to construct a planar transformer showing excellent electric properties even when operated with small power of up to 10 W.
The fabrication process according to the second aspect of the invention has the following advantages over a conventional process of fabricating planar coils.
(1) By using as the plating primer film the metal thin film provided all over the surface of the substrate, a large current can be so supplied that a high-aspect pattern can be formed and the given plating thickness can be obtained in one plating operation.
(2) All regions but the plating primer film under the openings in the conductor are covered with the resist, and the plating primer film in relatively thin. Accordingly, even when the conductor spacing is very narrow relative to the conductor height, high throughput can be achieved by wet etching yet at low running cost.
(3) The resist used for plating is a positive photoresist, and so the inter-conductor primer film can be selectively stripped away at post-treatment steps.
(4) The formed conductor itself can be used for exposure of the inter-conductor primer film for etching purposes. This can dispense with the use of a photomask, and awkward alignment, and so can simplify the fabrication process.
(5) The protective metal layer is usually of a few xcexcm in thickness, and so can be formed within a short time of about 10 minutes immediately upon plating. Thus, the fabrication process can be simplified.